The highway under Boston has turned out to be one of the better solutions that the area could've had, I think. It doesn't mean that the highway that goes around (I-95) is unused, or even underused, but I-93/the former Central Artery going underground has allowed some really important revitalization of parts of the city while also giving pretty direct and (outside of the worst part of rush hour) quick access to most of Boston and Cambridge by car.
As forward-looking as much of the area is, we weren't getting away with "less car", and I don't think most places will today either.
I was thinking about this too. I've never lived in Boston, but have visited often enough over the past 20-25 years. While the Big Dig was an awful, wasteful, corrupt project, the results -- IMO -- make it all worth it. Moving all those highways underground made quite a few areas much more livable than before.
As forward-looking as much of the area is, we weren't getting away with "less car", and I don't think most places will today either.